The invention relates to a transmission system for transmitting data signals at a given symbol rate 1/T through a channel in which additive pulse-like disturbances occur, which system comprises a data transmitter having a discrete-time smearing filter with a finite impulse response s(i) with i=0, 1, 2, . . . , N.sub.s -1, where N.sub.s represents the length of this impulse response, and a data receiver having a discrete-time smearing filter with a finite impulse response d(j) with j=0, 1, 2, . . . , N.sub.d -1, where N.sub.d represents the length of this impulse response, which smearing filters are substantially complementary and which impulse response d(j) can assume only coarsely quantized values.
Such a transmission system is known from an article entitled "Design of Smearing Filters for Data Transmission Systems" by G. F. M. Beenker et al, published in IEEE Trans. Commun., Vol. COM-33, No. 9, September 1985, pp. 955-963.
As described in the above article, smearing filters placed at the transmit and receive ends of the system are used for combatting the influence of concentrated pulse-like disturbances which can occur in the transmission channel at irregular instants that are usually rather far apart. Such pulse-like disturbances are generally referred to as impulse noise.
According to the design strategy for smearing filters discussed in this article, those filter impulse responses s(i) and d(j) are looked for, for which the performance as regards smearing efficiency and suppression of the intersymbol interference caused by the cascade combination of the two filters is optimized. In addition, it is noted that the implementation of these smearing filters can be simplified by avoiding complicated multiplying operations. This is allowed as according to the above article the impulse responses of the smearing filters can be defined by coarsely quantized coefficients, more specifically, binary or ternary coefficients, without thereby essentially impairing the performance of the smearing filters.
Recent developments in the field of data processing and data transmission have led to concepts of systems and implementations, in which data streams have to be transmitted at high rates. This is illustrated, for example, by the ISDN concept having a standardized net bit rate of 144 kbit/s for the user. For economic reasons, the implementation requires the use of already available transmission cables, such as, for example, the so-called quad-pair cables. In such a situation the wire pairs for conventional analog telephone traffic are housed in one and the same cable as the wire pairs intended for ISDN circuits. The aforesaid pulse-like disturbances can be injected into such ISDN circuits from adjacent wire pairs. Such pulse-like disturbances are especially annoying in those cases wherein such ISDN circuits range over relatively large distances. In this way, dialling pulses and other kinds of signalling pulses occurring in conventional analog telephone circuits can considerably impair the quality of adjacent ISDN circuits as a result of their high voltage values, such as 60-80 volts peak-to-peak.
As can be seen from an article entitled "Measured Performance of ISDN Transmission in the Dutch Local Telephone Network" by S. J. M. Tol and W. van der Bijl, published in Proc. ISSLS 86, Tokio, pp. 302-307, the above-mentioned pulse like disturbances to be combatted do not have any significant content above approximately 30 kHz and therefore these disturbances have a pronounced band-limited nature. The bandwidth of such pulse-like disturbances can be considerably smaller than the Nyquist bandwidth 1/(2T) of the data signals to be transmitted.
The invention has for its object to provide a transmission system of the type mentioned in the opening paragraph in which the aforesaid bandwidth-limited nature of the pulse-like disturbances is utilized to further simplify the implementation of smearing filters and/or to improve their performance level.
The transmission system according to the invention is characterized in that the non-zero values of the respective impulse responses s(i) and d(j) only occur at the points of grids of the respective indices i and j recurring with a period of L, where L is a factor representing an integer greater than 1 and where L is determined substantially by the ratio L.sub.o of the symbol rate 1/T to twice the highest significant frequency f.sub.m of the occurring pulse-like disturbances.
The invention provides a way of reducing the coefficient density of the filters roughly by the above factor L and thereby simplifying their implementation without affecting their performance level. Conversely the measures in accordance with the invention can be used to achieve an improved performance level with unchanged complexity.
The impulse response of a smearing filter in accordance with the invention solely has real values. This implies that only the part of the transfer function corresponding to the impulse response and situated between 0 and 1/(2T) in the fundamental interval of the data signals the size of 1/T can be selected freely. As, apparently, the pulse-like disturbances to be combatted in the local public telephone network do not have a significant spectral content above roughly 30 kHz, in the case of ISDN circuits a relatively large part of the fundamental interval of the data signals the size of 1/T is strictly speaking irrelevant to the smearing function, so that the fundamental interval of the transfer function can be reduced, more specifically, by the above L-factor. Therefore, the design of a smearing filter in accordance with the invention is not based on a fundamental interval equal to 1/T but on a fundamental interval equal to 1/(LT). Worded differently, within the band between 0 and 1/(2LT) the transfer function can be chosen at freely and due to its periodic character this function is fixed for all other frequencies.
It should be noted that the U.S. Pat. No. 4,476,539 discloses a transmission system comprising a smearing filter at the transmit end and a smearing filter at the receive end. According to this technique known per se the smearing filters are implemented as a tapped cascade combination of delay elements, each of which introduces a time delay of T. Herein T=1/(2F.sub.m), where F.sub.m represents the highest frequency of an information signal to be transmitted. In the above Patent it is likewise proposed to introduce a time delay mT between adjacent taps, where m is an integer greater than 2. With respect to the object and implementation, however, such smearing filters known per se are basically different from the smearing filters used in a transmission system in accordance with the invention. According to the known technique it is proposed to dimension a smearing filter implemented in the above way such that the group-delay characteristic has a sinusoidal shape. This implies that of necessity the filter coefficients have to be quantized finely, entailing a relatively large complexity of the filter implementation. In addition, the impulse response of the transmit-end smearing filter is required to be the time-inverse of the impulse response of the receive-end smearing filter. This imposes a restriction on the filter design, which restriction is absent for smearing filters as employed in a transmission system in accordance with the present invention.